Technical Field
The present invention relates to a microelectromechanical (MEMS) device having a driving structure of an electrostatic type.
Description of the Related Art
As is known, picoprojectors have been proposed having a pair of MEMS micromirrors, each driven for rotating about an own rotation axis.
For instance, in the system illustrated schematically in FIGS. 1 and 2, a light source 1, typically a laser source, generates a beam of light 2 (generally including three monochromatic beams, one for each basic color), which, through an optics 3 illustrated only schematically, is deflected by a pair of MEMS mirrors 5, 6. For instance, the first MEMS mirror 5 may be a horizontal micromirror, of a resonant type, which rotates about a first axis A (FIG. 2) and generates a fast horizontal scan, and the second MEMS mirror 6 may be a vertical micromirror, which rotates about a second, transverse, axis B, in particular perpendicular to the first axis A, and generates a slow vertical scan, typically a sawtooth scan. The combination of the movements of the two MEMS mirrors 5, 6 causes the beam of light 2 to perform a complete two-dimensional scanning movement and, once projected on a projection screen 7, to generate a two-dimensional image thereon. Such a system is described, for example, in WO 2010/067354, which was also published as US 2011/0234898, which is hereby incorporated by reference in its entirety for all purposes.
For ideal projection of the image, it is desired to perform the vertical scan at a constant rate. Consequently, it has already been proposed to control the movement at least of the vertical micromirror 6 by an electrostatic driving system that causes controlled rotation thereof about the respective axis B.
For instance, the vertical micromirror 6 may be made as illustrated in FIG. 3. Here, a die 10 comprises a reflecting surface 11 suspended on a substrate (not visible) and supported by a pair of arms 12. The arms 12 extend on opposite sides of the reflecting surface 11 and define the rotation axis B of the vertical micromirror 6. The arms 12 are connected to a fixed peripheral portion 13 of the die 10 via torsion springs 14 that enable rotation of the arms 12 about the axis B. The arms 12 are further coupled to a driving structure formed by two actuation assemblies 18 of an electrostatic type, one for each arm 12. Each actuation assembly 18 here comprises two sets of mobile electrodes 19 facing as many sets of fixed electrodes 20.
In detail, the mobile electrodes 19 are fixed with respect to the arms 12 and are comb-fingered with the fixed electrodes 20 for generating a capacitive coupling. The fixed electrodes 20 are carried by supporting regions 21, generally fixed with respect to the substrate (not visible) of the die 10. By virtue of the arrangement of the electrodes 19, 20, the driving structure is also defined as “comb drive structure”.
The mobile electrodes and the fixed electrodes 20 may be formed in different layers of the die 10 so as to extend at different heights, as described for example in WO 2010/137009, which was also published as US 2012/0062973, which is hereby incorporated herein by references in its entirety for all purposes, and as illustrated in the enlarged detail of FIG. 4.
By applying appropriate voltages between the mobile electrodes 19 and the fixed electrodes 20, it is possible to generate attraction or repulsion forces between them for causing a rotation of the mobile electrodes 19 with respect to the fixed electrodes 20, a torsion of the arms 12 about axis B, and thus a corresponding controlled rotation of the reflecting surface 11 (FIG. 5).
The described structure is in contrast, however, with the current demand for increasingly large mirrors.
In fact, in optical systems, the number of pixels of the images projected is proportional to the diameter of the reflecting surface 11.
It follows that, in the field of picoprojectors, it is desirable to have micromirrors of increasingly large dimensions, both in order to increase resolution of the projected image and to meet market demand, moving towards high-definition standards (720 pixels; 1080 pixels).
In particular, in two-dimensional picoprojectors, the diameter of the horizontal micromirror determines the resolution. On the other hand, the increase of the dimensions of the horizontal micromirror also determines a drastic increase of the dimensions of the vertical micromirror on account of the larger scanning-spot trace thereon.
To provide a reflecting surface 11 of larger dimensions, it is possible to use a higher actuation voltage, since the resonance frequency is set by the conditions and constraints of the system. In particular, in the case of electrostatic comb driving of the considered type, this entails a modification of the operating voltage and thus an increase in the number of electrodes.
With the structure of FIGS. 3-5, the increase in the dimensions of the reflecting surface 11 results in a significant increase in the dimensions of the entire die 10 in the direction of the rotation axis B, since not only the length of the reflecting surface 11 increases in this direction, but also the number of pairs of comb-fingered electrodes 19, 20.
The above increase is in contrast with the current trend towards miniaturization and entails considerable costs, in particular in the case of applications at a modest production scale, where it is not desired to exploit to the full the potential of mass production.
This problem exists, on the other hand, also in other types of MEMS devices, of the type referred to above.